The most common method of measuring the concentration of contaminants and other constituents in environmental media, such as water and air, involves collecting “grab samples” (i.e., the direct collection of a sample of the environmental media over a time period of minutes or less, such that it represents the concentration at an instant in time). Most of the currently available sampling methods (e.g., purge sampling; SNAP Sampler as in U.S. Pat. No. 7,178,415, HYDRASleeve as in U.S. Pat. Nos. 6,481,300 & 6,837,120; etc.) collect such grab samples.
However, grab samples can be significantly affected by short-term variability in the concentrations of contaminants in the environment. Short-term variability has been shown to primarily occur over a time scale of days to weeks or less, and can present as a seemingly random increase or decrease in contaminant concentrations unrelated to the long-term trends associated with ongoing impacts, natural contaminant attenuation, or site remediation. Because grab samples capture the concentration at an instant in time, rather than a period of time, the grab samples are particularly subject to the effects of short-term variability, making it difficult to observe meaningful trends in concentration over time.
As a result of the random variability associated with grab samples (i.e., individual grab samples commonly over-estimate or under-estimate the average concentration by a factor of 2×, and sometimes by as much as 10×), additional sample locations and more frequent sampling events are often utilized to better understand the overall trend in concentrations. However, increased monitoring can drive up costs, and may still yield incorrect conclusions. For example, even in a network of 10 or 20 monitoring wells used for measuring groundwater pollution, concerns may be raised after only one of the monitoring wells in the network shows increasing concentrations in one sampling event despite the fact that the likely explanation for this apparent increase in concentration is natural short-term variability.
To address these problems, the environmental monitoring field is moving away from these types of methods of obtaining multiple grab samples of an environmental media to more time-integrated sampling methods (i.e., sampling that occurs over a period of days, weeks, or months to provide an average concentration). Instead of collecting a direct sample of the environmental media (e.g., water or air), time-averaged sampling involves extraction and collection of the contaminant from the environmental media over time—thus reducing the volume of the sample needed, and extending the period of time in which a time-averaged concentration can be determined. In addition, by averaging the sampling over a longer period of time, the confounding influence of spurious data is reduced, and the actual time involved in observing the key environmental variable (such as concentrations in air, water, or sediment) is greatly increased. For example, taking two grab samples per year means that just two brief moments in time represent the entire year of concentrations in the environment. A three-month integrative sampling, however, can increase this temporal coverage. Also, the costs for performing time-integrated sampling in environmental media can be significantly less compared to conventional grab sampling methods because repetitive field trips to do the sampling are not needed.
Various examples of time-integrated samplers are available to measure concentrations of volatile organic compounds (VOCs). For instance, the 24-hr Summa canisters or 7-day passive sorbent samplers have been the default devices for measuring VOC concentrations in air. Various samplers that measure average concentrations of semi-volatiles in sediments over multi-month periods are also available. In groundwater, deployment periods for currently-available time-integrated samplers are constrained by the design of those samplers, where current groundwater samplers (e.g., AGI Universal Sampler aka Gore Sorber, Enviroflux, etc.) are typically deployed over a time period of only hours to 28 days, such as disclosed in U.S. Pat. No. 6,401,547 to Enviroflux.
Each of these devices is a form of “kinetic” sampler, where the sampler is designed to have a constant uptake rate throughout the sampling duration. Unfortunately, kinetic samplers can face limitations that compromise the accuracy of the measurements over an extended time period. Specifically, kinetic samplers are sensitive to changes in the flow velocity of the medium (e.g., air or water), which can alter the thickness of the sampler's boundary diffusion layer (comprised of a stagnant fluid layer in front of a membrane and the membrane itself). In turn, this sensitivity can affect the concentration's gradient across the layer and ultimately can affect the contaminant's mass that is trapped within the sampler (e.g., adsorbed to a sorbent). Consequently, uptake rates can vary in kinetic samplers if changes in the ambient fluid flow velocity change the thickness of the kinetic sampler's diffusion layer.
Also, kinetic samplers are subject to potential biofouling, degradation of their membranes and/or sorbents, or other detrimental processes that change the sampler's uptake rate or otherwise cause sampling errors. Moreover, these detrimental processes can limit the effective deployment time of the sampling device in certain settings.
For at least these reasons, a time-integrated sampler is needed that addresses these limitations and provides a dependable and effective method for the measurement of average constituent concentrations in a fluid over an extended period of time (i.e., months). Such a time-integrated sampler could potentially eliminate most short-term variability problems that now plague monitoring programs based on the individual grab samples. To that end, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.